1. Field of the Invention
The present invention relates generally to thermal management of RF power amplifiers of the type employed in wireless communications base stations. In particular the present invention relates to power amplifier assemblies and heat sink assemblies.
2. Description of the Prior Art and Related Background Information
Radio frequency (RF) power amplifiers of the type employed in cellular base stations typically comprise an RF amplifying circuit. The RF amplifying circuit generally is comprised of various devices including input and output impedance matching circuitry and a power amplifier device, typically one or more RF transistors. When the power amplifying device is operating, it generates a substantial amount of concentrated energy in form of heat. Proper thermal management is imperative to avoid degraded performance or permanent damages to devices. Any degradation in performance introduces distortion into the communication signal which is highly undesirable. Therefore, maintaining lower temperature for devices is a key issue in RF power amplifier design.
To address such thermal management issues, a main mechanical element of a packaged RF amplifier assembly is a finned metal heat sink. This heat sink is normally constructed from aluminum due to the reasonable economics of the material along with favorable thermal conductivity. The heat sink hosts a variety of electronic devices to provide mechanical support and more importantly serves as a cooling element for heat generating devices. In the presence of conventional convection methods, the thermal properties of aluminum and the general design of finned structures are satisfactory to meet the cooling requirements of the majority of devices. For RF devices used in power amplifiers, however, the heat generation density and large aspect ratio of sink to source creates notable resistance against conduction spreading. To improve this condition, the amplifier portion of the electronics is mounted on a copper heat spreader known as a pallet to reduce the conduction spreading resistance due to copper's superior thermal conductivity. Pallets are solid flat plates and they enhance the conduction path from the small source of heat generation to the main aluminum heat sink. Pallets also accommodate a modular design approach for building different designs of amplifiers. Copper pallets are rather costly and heavy, however. Also, depending on the configuration of the amplifier, it is not unlikely to have a few pallets in each amplifier which drives up the cost and weight. Thus, it is desirable to find alternatives to copper pallets.
An aluminum pallet would reduce the cost and weight as desired. However, due to aluminum's lower thermal conductivity than copper, there would exist a larger spreading resistance, which leads to higher device temperature that is not desirable for amplifier operation. This becomes a significant problem in high power RF amplifier applications. The high-density energy generation in each RF amplifier device utilized in power amplifiers for wireless telecommunications could reach as high as 100-150 W. Such high heat fluxes in small packages complicate the thermal management solutions. Therefore, for high power RF amplifiers substituting the copper pallet with an aluminum pallet leads to poor temperature distribution in the pallet which ultimately causes higher device temperature.
Accordingly, a need presently exists for a heat transfer system for RF power amplifiers capable of providing the necessary heat transfer from high power RF devices while avoiding the cost and weight of copper pallets.